1. Field of the Invention
The present invention relates to a surface acoustic wave filter device having a cascade-connected structure including a plurality of longitudinally coupled resonator type surface acoustic wave filter units and a duplexer including the surface acoustic wave filter device and, in particular, to a surface acoustic wave filter device having a surface acoustic wave resonator electrically connected between stages thereof and a duplexer including the surface acoustic wave filter device.
2. Background of the Related Art
A surface acoustic wave filter device is commonly used as a bandpass filter in an RF stage of a cellular phone. There is a high demand for this type of surface acoustic wave filter device to support a high-frequency operation, and for a balanced-unbalanced transform function to be incorporated in the surface acoustic wave filter device.
Recently, a frequency bandwidth used in cellular phones has increased, and a frequency band on a transmission side and a frequency band on a reception side are closer to each other. A bandpass filter must have a wide passband and high frequency selectivity, and more specifically, the bandpass filter must have sharp filter characteristics.
To increase the sharpness of the filter characteristics, a technique of connecting surface acoustic wave resonators, in series or in parallel, with longitudinally coupled resonator type surface acoustic wave filters is commonly used in the conventional art.
In the configuration in which a surface acoustic wave resonator is connected in series with a longitudinally coupled resonator type surface acoustic wave filter, sharpness at the high frequency side of the passband is increased by setting a resonance point of the surface acoustic wave resonator in a passband of the filter and by setting an anti-resonance point of the surface acoustic wave resonator in an attenuation region in the vicinity of a high frequency end of the filter passband.
In contrast, in a configuration in which a surface acoustic wave resonator is in parallel with a longitudinally coupled resonator type surface acoustic wave filter, sharpness at the low frequency side of the passband is increased by setting an anti-resonance point of the surface acoustic wave resonator within the filter passband and by setting a resonance point of the surface acoustic wave resonator in an attenuation region in the vicinity of a low frequency edge of the filter passband.
Whether the surface acoustic wave resonator is to be connected to an input terminal or an output terminal of the longitudinally coupled resonator type surface acoustic wave filter may be determined based on the impedances of the input terminal and the output terminal.
On the other hand, in a configuration in which two stages of longitudinally coupled resonator type surface acoustic wave filters are cascade-connected, a surface acoustic wave resonator may be connected between the two surface acoustic wave filters, i.e., between the stages. Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 4-54011 discloses a longitudinally coupled resonator type surface acoustic wave filter device having a two cascade-connected structure with a surface acoustic wave resonator connected between the stages.
FIG. 12 is a plan view schematically illustrating a surface acoustic wave filter device described in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 4-54011.
A surface acoustic wave filter device 1001 includes an electrode structure arranged on a piezoelectric substrate 1002. The surface acoustic wave filter device 1001 includes an input terminal 1003 and an output terminal 1004. First and second surface acoustic wave filter units 1005 and 1006 are connected between the input terminal 1003 and the output terminal 1004.
The first and second surface acoustic wave filter units 1005 and 1006 respectively include first and second IDTs 1005a and 1005b and first and second IDTs 1006a and 1006b arranged to be adjacent to each other in a surface acoustic wave propagation direction. Reflectors 1005c and 1005d are arranged on both sides of an area in which the IDTs 1005a and 1005b are arranged in the surface acoustic wave propagation direction. Similarly, reflectors 1006c and 1006d are arranged on both sides of an area in which IDTs 1006a and 1006b are arranged in the surface acoustic wave propagation direction.
One end of the first IDT 1005a of the first surface acoustic wave filter unit 1005 is connected to the input terminal 1003. On the other hand, one end of the second IDT 1006b of the second surface acoustic wave filter unit 1006 is connected to the output terminal 1004. One end of the second IDT 1005b is connected to one end of the first IDT 1006a via a signal line 1007, and the first and second surface acoustic wave filter units 1005 and 1006 are thus electrically connected to each other. The other end of each of IDTs 1005a, 1005b, 1006a, and 1006b is connected to the ground potential.
In the surface acoustic wave filter device 1001 in which the first and second surface acoustic wave filter units 1005 and 1006 are two-stage cascade-connected, a surface acoustic wave resonator 1008 is connected between the signal line 1007 and the ground potential. One end of the surface acoustic wave resonator 1008 is connected to the signal line 1007 and the other end of the surface acoustic wave resonator 1008 is connected to the ground potential. More specifically, the surface acoustic wave resonator 1008 between the stages is connected between the signal line and the ground potential.
The amount of attenuation is increased in the attenuation region of the high frequency side of the passband by setting the resonance point of the surface acoustic wave resonator 1008 in the attenuation region of the high frequency side of the passband. More specifically, the surface acoustic wave resonator 1008 is connected in the surface acoustic wave filter device 1001 described in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 4-54011 not for the purpose of increasing the sharpness on the low frequency side of the passband but for the purpose of increasing the amount of attenuation in the attenuation region of the high frequency side of the passband.
In the surface acoustic wave filter device 1001 described in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 4-54011, the surface acoustic wave resonator 1008 is connected in parallel, and more specifically in parallel with the IDTs 1005b and 1006b in an inter-stage portion, and one end of the surface acoustic wave resonator 1008 is connected to the ground potential. For this reason, a capacitance of the inter-stage portion increases with respect to the ground potential, which causes the passband to become narrow. Thus, the surface acoustic wave filter device cannot sufficiently satisfy wide band requirements.